The first law of thermodynamics

Models

Models are not reality they are just pictures to help us understand things

Different types of working

You can have mechanical work , this is where forces move masses, or electrical work , this is where forces move charges.

Extra

Δ.... just means " change in....."

In our model when you lose money by working the amount you have gained is = -W

when you lose money because of differences in wealth, the amount you have gained is= -Q

A model for getting your head around the first law of thermodynamics

You will find different versions of the 1st law on the net or in books. They are practically the same. We are going to use
ΔU= Q +W and a model to help us understand the first law.

You may already know that U has something to do with internal energy , Q is something to do with heating, and W is something to do with working. Here is a model

Let's imagine that U is the amount of money you have saved. Its like a bank account or your own personal store of money. You dont know its exact value because its just too time consuming to add all the different amounts up. There are 2 different ways of changing the amount of money you have. The first way is by working.

Imagine a world in which when you do work (like moving something) you end up spending some of your money. And if someone decides to do work on you (i.e moving you or part of you from one place to another ) you gain some money from them. The amount of money you gain in this way is called W. In this world you can also gain or lose money without any work taking place. It works like this.

If you are in contact with people who have more money that you (i.e more U than you) some of thier money drifts into your account (it could be because of taxes, theft or whatever). Of course, the opposite is true as well, if you are in contact with people with less money than you, some of your money (U) drifts out of your account. The amount of money you gained in this way is called Q.

Now let's imagine that you are in contact with someone who has the same amount of U as you. You decide to do some work on that person. What will happen to your U? It will decrease. You don't know your new value of U because you dont know how much money you had to start with. But you do know how much U has decreased by (ΔU). You also know that Q is 0. No money has been tranferred due to differences in wealth. So the 1st law equation becomes ΔU = W

Now lets say that you do no work , but that you are in contact with wealthly people. Q will increase, W = 0, theferore the 1st law equation becomes ΔU = Q

How does this model relate to A physics exam questions

If you imagine money to be energy, then U, Q and W are all measured in joules. Differences in wealth are just differences in temperature. Work is just work. This model will help you figure out how to apply the first law of thermodynamics to different situations.

(c) The Undercover Physicist 2007.